The major objectives of our research endeavors are to examine two macrophage functions, cytostasis and cytolysis, that adversely affect tumor cells to determine whether they are independent events caused by macrophages at different stages of activation and/or by different macrophage subpopulations. The experimental approaches will involve both the comparison of cytostatic and cytolytic activities with respect to a variety of parameters known to affect macrophage activities. Activated macrophages can recognize, bind to, and lyse tumor cells in an antibody-independent manner. We have found that tumor cells pretreated with phorbol esters are no longer susceptible to macrophage-mediated cytolysis, although the initial binding step is unaffected. The non-promoting derivatives, 4-beta-phorbol and 4LPDD, were inactive in protecting against cytolysis, and TPA did not protect tumor cells from lysis by antibody plus complement. Since the phorbol ester receptor has been identified as protein kinase C in other cells, a search was initiated for substrates phosphorylated only in the presence of phorbol esters. Preliminary findings indicate that proteins of Mr = 47, 60, and 140 kilodaltons are substrates for TPA-induced phosphorylation. This suggests a possible mechanism for the resistance of phorbol ester-treated tumor cells to macrophage-mediated cytolysis. (MB)